The invention relates to a piezoelectric sensor according to the preamble of patent claim 1 for an improved measurement of mechanical variables such as force, pressure or measurement variables which are derived there from, particularly a PVDF film sensor exhibiting an improved sensitivity and temperature stability of the measurement signal for measuring pressures that vary in time and/or space, and for the one- and two-dimensional determination of a position and propagation velocity of pressure fluctuations and pressure waves. A preferred field of application for the invention is a non-invasive, low impact and continuous measurement of a pulse rate and a systolic and diastolic blood pressure of humans and animals by measuring arterial pulse waves through a sensor configured with piezoelectric PVDF foils.
Through the oriented deformation of a piezoelectric material in a direction of a polarization, microscopic dipoles are formed by displacing negative and positive charge centers within the elementary cells. The summation over all elementary cells of the crystal leads to a macroscopically measurable electrical voltage, which is directly proportional to a deformation in a defined deformation range for a longitudinal deformation in a direction of a polarity.
Piezoelectric foils are typically made from polyvinylidenfluoride (abbrev. PVDF). A transparent, partially crystalline fluoride thermoplastic, which is polarized for producing the piezoelectric properties, this means it is heated, stretched in one dimension and thus exposed to a strong oriented electromagnetic field for orienting the dipoles. The piezoelectric properties thus created are highly directional through the monoaxial orientation. In order to discharge the dipole charges created during the deformations, the PVDF foil is metal coated. This metal coating is mostly made from gold or copper nickel alloys, vapor deposited on the foil. Thus, the charges can be tapped through electrodes at the edge of the piezoelectric foil and can be converted into a measurable electric current through a charge amplifier (charge-voltage converter).